1. Field of the Invention
The present invention relates to a semiconductor device having a microstructure such as a micromachine, and also relates to a manufacturing method thereof.
2. Description of the Related Art
Researches have been conducted on manufacturing minute machines by a process for manufacturing a semiconductor element and a method similar to that process. For example, a three-dimensional structural body having a space and a portion moving in the space, such as a gear or a bridge structure, is manufactured by processing a silicon wafer.
In recent years, researches have been advanced on manufacturing a micro electromechanical device having one function by manufacturing a structural body as aforementioned and a semiconductor element over one substrate or by attaching a structural body and a semiconductor element, which are manufactured separately, to each other. The electromechanical device is referred to as a MEMS (Micro Electro Mechanical System), a micromachine, or the like. At present, micromachines such as the following are manufactured: a pressure sensor or an acceleration sensor that detects movement of a portion thereof which moves in a space; a variable capacitor, an inductor, or a waveguide which uses a space as a favorable insulator other than having a portion moving in the space; and the like.
Micromachines are classified into two groups according to their manufacturing methods. One is called bulk micromachines which are obtained by manufacturing a three-dimensional structural body in such a way that a silicon wafer or an SOI substrate itself is processed by etching or polishing. The other is called surface micromachines which are obtained by manufacturing a three-dimensional structural body in such a way that a thin film is stacked over a substrate such as a silicon wafer and the thin film is processed by photolithography, which is a general process for manufacturing a semiconductor element.
Since a bulk micromachine is obtained by processing a substrate from various directions, the degree of freedom in manufacturing a three-dimensional structural body is so high that various structural bodies can be manufactured. However, many steps are different from those in a process for manufacturing a semiconductor element. Accordingly, in many cases, a semiconductor element and a structural body are separately manufactured and then they are attached to each other or they are packaged into one without being attached to each other.
Here, a basic method for manufacturing a structural body in a surface micromachine is described. First, as shown in FIG. 27A, a sacrifice layer 2701 is formed to form a space in a structural body. This sacrifice layer 2701 is to be removed later. Next, as shown in FIG. 27B, a structural layer 2702 is formed over the sacrifice layer. The structural layer has a three-dimensional shape because the structural layer is formed over the sacrifice layer. Then, a space 2703 is formed under the structural layer by etching away the sacrifice layer 2701 as shown in FIG. 27C; thus, a part of the structural layer over the space (here a portion above the sacrifice layer) can move.
Here, an example of a simple structural body having the structural layer 2702 and the space 2703 over a substrate is shown.
As well as being an important step in manufacturing a micromachine, sacrifice layer etching is a rate-controlling point having a number of problems. For example, sacrifice layer etching needs to progress in a lateral direction, i.e., a direction parallel to a substrate surface in order to remove a part of the sacrifice layer that overlaps with the structural layer, and thus, the sacrifice layer etching is conducted by isotropic etching. Moreover, since the sacrifice layer has thick thickness in order to form a space and is as long as several micrometers, it takes much time to etch the sacrifice layer. Further, the sacrifice layer etching leads to problems in that the sacrifice layer 2704 is left because a part thereof that overlaps with the structural layer 2702 cannot be removed completely as shown in FIG. 28A, and that the structural layer buckles as shown in FIG. 28B and attaches the substrate.
In such a surface micromachine, it is necessary that the sacrifice layer be formed in order to form a space and a structure portion that can move in the space, and that sacrifice layer etching be performed to remove the sacrifice layer. This sacrifice layer etching is a significant rate-controlling point in manufacturing a micromachine. This is because it takes much time to isotropically etch the sacrifice layer that basically has thick thickness and moreover because, in many cases, the movable portion and the substrate are attached to each other after the etching.
For example, in order to shorten the time for the sacrifice layer etching, a process in which a tunnel is provided in the sacrifice layer so that an etchant is easily introduced in the sacrifice layer at the sacrifice layer etching is considered (see Patent Document 1).
The attachment between a bottom surface of the movable portion and a surface of the substrate includes attachment caused when drying after the sacrifice layer etching and attachment caused as a secondary result of impact or static electricity after the drying. It is considered that the attachment when drying is caused by sublimation or supercritical drying and the secondary attachment is caused by formation of a low-energy film.    [Patent Document 1] Japanese Published Patent Application No. 2000-58866
According to the method described in Patent Document 1, the time required for the sacrifice layer etching can be shortened. However, since the sacrifice layer needs to be formed in two separate steps, the number of steps is increased overall. A countermeasure against the attachment caused when drying and the countermeasure disclosed in Patent Document 1 to prevent the secondary attachment both have problems in long-term reliability and require particular apparatuses and materials.
Although micromachines have functions that cannot be realized by semiconductor elements, micromachines have not yet been introduced into markets because cost cannot be reduced due to its complicated process. Thus, inexpensive and high-value-added micromachines are expected. As a specific example, manufacturing of a plurality of structural bodies with different functions over one substrate through the same process is expected.